With YAM 2.8 you now have the concept of "identities". That means, you are now supposed to create an identity for each email address you want to be able to send mail from. All you have to do is to go to the "Identity" configuration pane in the YAM configuration window and setup a second or more identities. As soon as two identities are setup YAM will show you a cycle gadget in the write window which will allow you to select the identity you want the email to be sent from.

Besides this new cycle gadget, the new "identity" mechanism will allow you to automatically select a different signature per identity or select a different sent folder or even select a different SMTP server if you use more than one provider for sending emails.